Catalytic reduction of NOx

ABSTRACT

A system for NO x  reduction in combustion gases, especially from diesel engines, incorporates an oxidation catalyst to convert at least a portion of NO to NO 2 , particulate filter, a source of reductant such as NH 3  and an SCR catalyst. Considerable improvements in NO x  conversion are observed.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.13/204,634, filed Aug. 5, 2011, which was a continuation of U.S.application Ser. No. 12/380,414, filed Feb. 27, 2009, which is acontinuation of U.S. application Ser. No. 10/886,778, filed Jul. 8,2004, which is a divisional application of U.S. application Ser. No.09/601,964, filed Jan. 9, 2001, now U.S. Pat. No. 6,805,849, which isthe U.S. National Phase of International Application No. PCT/GB1999/000292, filed Jan. 28, 1999, and which claims the benefit ofpriority from British Application No. 9802504.2, filed Feb. 6, 1998.These applications, in their entirety, are incorporated herein byreference.

SUMMARY OF THE INVENTION

The present invention concerns improvements in selective catalyticreduction of NO_(x) in waste gas streams such as diesel engine exhaustsor other lean exhaust gases such as from gasoline direct injection(GDI).

BACKGROUND OF THE INVENTION

The technique named SCR (Selective Catalytic Reduction) is wellestablished for industrial plant combustion gases, and may be broadlydescribed as passing a hot exhaust gas over a catalyst in the presenceof a nitrogenous reductant, especially ammonia or urea. This iseffective to reduce the NO_(x) content of the exhaust gases by about20-25% at about 250° C., or possibly rather higher using a platinumcatalyst, although platinum catalysts tend to oxidise NH₃ to NO_(x)during higher temperature operation. We believe that SCR systems havebeen proposed for NO_(x) reduction for vehicle engine exhausts,especially large or heavy duty diesel engines, but this does requireon-board storage of such reductants, and is not believed to have metwith commercial acceptability at this time.

We believe that if there could be a significant improvement inperformance of SCR systems, they would find wider usage and may beintroduced into vehicular applications. It is an aim of the presentinvention to improve significantly the conversion of NO_(x) in a SCRsystem, and to improve the control of other pollutants using a SCRsystem.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph plotting percentage NO_(x) conversion againsttemperature resulting from Test 1.

FIG. 2 is a graph plotting percentage NO_(x) conversion againsttemperature resulting from Test 2.

FIG. 3 is a graph plotting percentage NO_(x) conversion againsttemperature resulting from Test 3.

FIG. 4 is a bar graph showing percentage conversion of pollutants[NO_(x), particulates, hydrocarbons (HC) and carbon monoxide (CO)]resulting from Test 4.

DETAILED DESCRIPTION OF THE INVENTION

Accordingly, the present invention provides an improved SCR catalystsystem, comprising in combination and in order, an oxidation catalysteffective to convert NO to NO₂, a particulate filter, a source ofreductant fluid and downstream of said source, an SCR catalyst.

The invention further provides an improved method of reducing NO_(x) ingas streams containing NO and particulates comprising passing such gasstream over an oxidation catalyst under conditions effective to convertat least a portion of NO in the gas stream to NO₂, removing at least aportion of said particulates, adding reductant fluid to the gas streamcontaining enhanced NO₂ to form a gas mixture, and passing the gasmixture over an SCR catalyst.

Although the present invention provides, at least in its preferredembodiments, the opportunity to reduce very significantly the NO_(x)emissions from the lean (high in oxygen) exhaust gases from diesel andsimilar engines, it is to be noted that the invention also permits verygood reductions in the levels of other regulated pollutants, especiallyhydrocarbons and particulates.

The invention is believed to have particular application to the exhaustsfrom heavy duty diesel engines, especially vehicle engines, e.g., truckor bus engines, but is not to be regarded as being limited thereto.Other applications might be LDD (light duty diesel), GDI, CNG(compressed natural gas) engines, ships or stationary sources. Forsimplicity, however, the majority of this description concerns suchvehicle engines.

We have surprisingly found that a “pre-oxidising” step, which is notgenerally considered necessary because of the low content of CO andunburnt fuel in diesel exhausts, is particularly effective in increasingthe conversion of NO_(x) to N₂ by the SCR system. We also believe thatminimising the levels of hydrocarbons in the gases may assist in theconversion of NO to NO₂. This may be achieved catalytically and/or byengine design or management. Desirably, the NO₂/NO ratio is adjustedaccording to the present invention to the most beneficial such ratio forthe particular SCR catalyst and CO and hydrocarbons are oxidized priorto the SCR catalyst. Thus, our preliminary results indicate that for atransition metal/zeolite SCR catalyst it is desirable to convert all NOto NO₂, whereas for a rare earth-based SCR catalyst, a high ratio isdesirable providing there is some NO, and for other transitionmetal-based catalysts gas mixtures are notably better than eithersubstantially only NO or NO₂. Even more surprisingly, the incorporationof a particulate filter permits still higher conversions of NO_(x).

The oxidation catalyst may be any suitable catalyst, and is generallyavailable to those skilled in art. For example, a Pt catalyst depositedupon a ceramic or metal through-flow honeycomb support is particularlysuitable. Suitable catalysts are, e.g., Pt/Al₂O₃ catalysts, containing1-150 g Pt/ft³ (0.035-5.3 g Pt/liter) catalyst volume depending on theNO₂/NO ratio required. Such catalysts may contain other componentsproviding there is a beneficial effect or at least no significantadverse effect.

The source of reductant fluid conveniently uses existing technology toinject fluid into the gas stream. For example, in the tests for thepresent invention, a mass controller was used to control supply ofcompressed NH₃, which was injected through an annular injector ringmounted in the exhaust pipe. The injector ring had a plurality ofinjection ports arranged around its periphery. A conventional dieselfuel injection system including pump and injector nozzle has been usedto inject urea by the present applicants. A stream of compressed air wasalso injected around the nozzle; this provided good mixing and cooling.

The reductant fluid is suitably NH3, but other reductant fluidsincluding urea, ammonium carbamate and hydrocarbons including dieselfuel may also be considered. Diesel fuel is, of course, carried on boarda diesel-powered vehicle, but diesel fuel itself is a less selectivereductant than NH₃ and is presently not preferred.

Suitable SCR catalysts are available in the art and include Cu-based andvanadia-based catalysts. A preferred catalyst at present is aV₂O₅/WO₃/TiO₂ catalyst, supported on a honeycomb through-flow support.Although such a catalyst has shown good performance in the testsdescribed hereafter and is commercially available, we have found thatsustained high temperature operation can cause catalyst deactivation.Heavy duty diesel engines, which are almost exclusively turbocharged,can produce exhaust gases at greater than 500° C. under conditions ofhigh load and/or high speed, and such temperatures are sufficient tocause catalyst deactivation.

In one embodiment of the invention, therefore, cooling means is providedupstream of the SCR catalyst. Cooling means may suitably be activated bysensing high catalyst temperatures or by other, less direct, means, suchas determining conditions likely to lead to high catalyst temperatures.Suitable cooling means include water injection upstream of the SCRcatalyst, or air injection, for example utilizing the engineturbocharger to provide a stream of fresh intake air by-passing theengine. We have observed a loss of activity of the catalyst, however,using water injection, and air injection by modifying the turbochargerleads to higher space velocity over the catalyst which tends to reduceNO conversion. Preferably, the preferred SCR catalyst is maintained at atemperature from 160° C. to 450° C.

We believe that in its presently preferred embodiments, the presentinvention may depend upon an incomplete conversion of NO to NO₂.Desirably, therefore, the oxidation catalyst, or the oxidation catalysttogether with the particulate trap if used, yields a gas stream enteringthe SCR catalyst having a ratio of NO to NO₂ of from about 4:1 to about1:3 by volume, for the commercial vanadia-type catalyst. As mentionedabove, other SCR catalysts perform better with different NO/NO₂ ratios.We do not believe that it has previously been suggested to adjust theNO/NO₂ ratio in order to improve NO reduction.

The present invention incorporates a particulate trap downstream of theoxidation catalyst. We discovered that soot-type particulates may beremoved from a particulate trap by “combustion” at relatively lowtemperatures in the presence of NO₂. In effect, the incorporation ofsuch a particulate trap serves to clean the exhaust gas of particulateswithout causing accumulation, with resultant blockage or back-pressureproblems, whilst simultaneously reducing a proportion of the NOR.Suitable particulate traps are generally available, and are desirably ofthe type known as wall-flow filters, generally manufactured from aceramic, but other designs of particulate trap, including woven knittedor non-woven heat-resistant fabrics, may be used.

It may be desirable to incorporate a clean-up catalyst downstream of theSCR catalyst, to remove any NH₃ or derivatives thereof which could passthrough unreacted or as by-products. Suitable clean-up catalysts areavailable to the skilled person.

A particularly interesting possibility arising from the presentinvention has especial application to light duty diesel engines (car andutility vehicles) and permits a significant reduction in volume andweight of the exhaust gas after-treatment system, in a suitableengineered system.

EXAMPLES

Several tests have been carried out in making the present invention.These are described below, and are supported by results shown ingraphical form in the attached drawings.

A commercial 10 liter turbocharged heavy duty diesel engine on atest-bed was used for all the tests described herein.

Test 1 (Comparative)

A conventional SCR system using a commercial V₂O₅/WO₃/TiO₂ catalyst, wasadapted and fitted to the exhaust system of the engine. NH₃ was injectedupstream of the SCR catalyst at varying ratios. The NH₃ was suppliedfrom a cylinder of compressed gas and a conventional mass flowcontroller used to control the flow of NH₃ gas to an experimentalinjection ring. The injection ring was a 10 cm diameter annular ringprovided with 20 small injection ports arranged to inject gas in thedirection of the exhaust gas flow. NO_(x) conversions were determined byfitting a NO_(x) analyser before and after the SCR catalyst and areplotted against exhaust gas temperature in FIG. 1. Temperatures werealtered by maintaining the engine speed constant and altering the torqueapplied.

A number of tests were run at different quantities of NH₃ injection,from 60% to 100% of theoretical, calculated at 1:1 NH₃/NO and 4:3NH₃/NO₂. It can readily be seen that at low temperatures, correspondingto light load, conversions are about 25%, and the highest conversionsrequire stoichiometric (100%) addition of NH₃ at catalyst temperaturesof from 325 to 400° C., and reach about 90%. However, we have determinedthat at greater than about 70% of stoichiometric NH₃ injection, NH₃slips through the SCR catalyst unreacted, and can cause furtherpollution problems.

Test 2 (Comparative)

The test rig was modified by inserting into the exhaust pipe upstream ofthe NH₃ injection, a commercial platinum oxidation catalyst of 10.5 inchdiameter and 6 inch length (26.67 cm diameter and 15.24 cm length)containing log Pt/ft³ (=0.35 g/liter) of catalyst volume. Identicaltests were run, and it was observed from the results plotted in FIG. 2,that even at 225° C., the conversion of NO_(x) has increased from 25%to >60%. The greatest conversions were in excess of 95%. No slippage ofNH₃ was observed in this test nor in the following test.

Test 3

The test rig was modified further, by inserting a particulate trapbefore the NH₃ injection point, and the tests run again under the sameconditions at 100% NH₃ injection and a space velocity in the range40,000 to 70,000 hr⁻¹ over the SCR catalyst. The results are plotted andshown in FIG. 3. Surprisingly, there is a dramatic improvement in NO_(x)conversion, to above 90% at 225° C., and reaching 100% at 350° C.Additionally, of course, the particulates, which are the most visiblepollutant from diesel engines, are also controlled.

Test 4

An R49 test with 80% NH₃ injection was carried out over a V₂O₅/WO₃/TiO₂SCR catalyst. This gave 67% particulate, 89% HC and 87% NO_(x)conversion; the results are plotted in FIG. 4.

Additionally tests have been carried out with a different diesel engine,and the excellent results illustrated in Tests 3 and 4 above have beenconfirmed.

The results have been confirmed also for a non-vanadium SCR catalyst.

We claim:
 1. A system for removing particulate matter and NOx from exhaust gas from a diesel engine, the system comprising: an oxidation catalyst configured to receive the exhaust gas and convert NO in the exhaust gas to NO2 to create an adjusted gas stream; a particulate trap downstream of the oxidation catalyst configured to receive the adjusted gas stream and trap the particulate matter in the adjusted gas stream to create a further adjusted gas stream; a reductant fluid injector downstream of the particulate trap configured to inject reductant fluid comprising urea into the further adjusted gas stream to create a further adjusted gas stream mixed with reductant fluid; and an SCR catalyst downstream of the reductant fluid injector configured to receive the further adjusted gas stream mixed with reductant fluid to create a final gas stream with a reduced NOx content by volume relative to the NOx content by volume of the exhaust gas; wherein the oxidation catalyst is further configured to convert enough NO in the exhaust gas to NO2 such that the adjusted gas stream exiting the oxidation catalyst (1) causes a significant portion of particulate matter in the particulate trap to combust such that there is no significant accumulation of particulate matter in the particulate trap at a temperature lower than a temperature that the particulate matter in the particulate trap would combust in the presence of the exhaust gas in an identical system without an oxidation catalyst such that there is no significant accumulation of particulate matter in the particulate trap, and (2) has an NO to NO2 ratio adjusted for greater NOx reduction over the SCR catalyst taking into account the decrease in NO2 caused by combustion of particulate matter in the particulate trap, relative to a lower NOx reduction over an SCR catalyst in an identical system without an oxidation catalyst.
 2. The system of claim 1, wherein the diesel engine is a vehicle engine.
 3. The system of claim 1, wherein the diesel engine is a heavy duty diesel truck engine.
 4. The system of claim 1, wherein the diesel engine is a turbocharged heavy duty diesel truck engine.
 5. The system of claim 4, wherein the turbocharger is configured to cool the further adjusted gas stream.
 6. The system of claim 1, wherein the oxidation catalyst is configured to convert less than all of the NO in the exhaust gas to NO₂.
 7. The system of claim 4, wherein the SCR catalyst is configured to convert more than 90% of the NO_(x) in the further adjusted gas stream mixed with reductant fluid when the further adjusted gas stream mixed with reductant fluid is at least 225° C.
 8. The system of claim 7, wherein the system is configured to create a final gas stream with 67% less particulate matter than the exhaust gas.
 9. A system for removing particulate matter and NOx from exhaust gas from a diesel engine, the system comprising: an oxidation catalyst configured to receive the exhaust gas and convert NO in the exhaust gas to NO2 to create an adjusted gas stream; a particulate trap downstream of the oxidation catalyst configured to receive the adjusted gas stream and trap the particulate matter in the adjusted gas stream to create a further adjusted gas stream; a reductant fluid injector downstream of the particulate trap configured to inject reductant fluid comprising urea into the further adjusted gas stream to create a further adjusted gas stream mixed with reductant fluid; and an SCR catalyst downstream of the reductant fluid injector configured to receive the further adjusted gas stream mixed with reductant fluid to create a final gas stream with a reduced NOx content by volume relative to the NO, content by volume of the exhaust gas; wherein the oxidation catalyst is further configured to convert enough NO in the exhaust gas to NO2 such that the adjusted gas stream exiting the oxidation catalyst (1) causes particulate matter in the particulate trap to combust at a temperature lower than a temperature that the particulate matter in the particulate trap would combust in the presence of the exhaust gas in an identical system without an oxidation catalyst, and (2) has an NO to NO2 ratio adjusted for greater NOx reduction over the SCR catalyst taking into account the decrease in NO2 caused by combustion of particulate matter in the particulate trap, relative to a lower NOx reduction over an SCR catalyst in an identical system without an oxidation catalyst; and wherein the system is configured to create a final gas stream with (1) 90% less NOx by volume than the exhaust gas when the further adjusted gas stream mixed with reductant fluid is at least 225° C., and (2) 67% less particulate matter by volume than the exhaust gas.
 10. The system of claim 9, wherein the diesel engine is a vehicle engine.
 11. The system of claim 9, wherein the diesel engine is a heavy duty diesel truck engine.
 12. The system of claim 9, wherein the diesel engine is a turbocharged heavy duty diesel truck engine.
 13. The system of claim 12, wherein the turbocharger is configured to cool the further adjusted gas stream.
 14. The system of claim 9, wherein the oxidation catalyst is configured to convert less than all of the NO in the exhaust gas to NO₂. 